ballcock (thing)

In an ordinary flush toilet, in the water tank behind the commode, the ballcock is the large, buoyant ball that floats and regulates the amount of water refilling the tank.

As it floats up, it presses on the water valve through a lever system, slowing down and eventually stopping the water. When the tank is empty/flushed, the ballcock sinks to the bottom, and opens the water valve. I suggest you open your tank and take a look.